ui: aggregate error/warning table and corresponding admin UI

[spencerkimball]

Have logger store error and warning level log messages to a system table.
Provide a corresponding admin UI for viewing aggregate system warning+ log messages.

[petermattis]

I'm not sure if our log.{Warning,Error} messages have had enough consideration to be displayed as-is in the admin UI. Now that we have the event-log facility, I think we should make a pass over the log.{Warning,Error} occurrences and consider upgrading some of them to events.

[petermattis]

Punting this until after beta.

[maxlang]

@mrtracy I'm going to assign to you as this seems related to #3725

[spencerkimball]

@mrtracy seems we can close this. I don't believe this is necessary as it hasn't proven to be a pressing ask since it was filed 1.5 years ago. Re-open if you disagree.